The high intensity sodium vapor lamps with which the invention is most useful comprise a slender tubular ceramic arc tube which is generally mounted in an outer vitreous envelope or glass jacket. The arc tube is made of light-transmissive refractory oxide material resistant to sodium at high temperatures, suitably high density polycrystalline alumina or synthetic sapphire. The filling comprises sodium together with mercury for improved efficiency, along with a rare gas to facilitate starting. The ends of the tube are sealed by closure members through which connections are made to thermionic electrodes which may comprise a tungsten coil activated be electron emissive material. The outer envelope which encloses the ceramic arc tube is generally provided at one end with a screw base to which the electrodes of the arc tube are connected.
The high pressure sodium vapor lamp contains an excess amount of sodium mercury amalgam, that is it contains more amalgam than is vaporized when the lamp reaches a stable operating condition. By having an excess, the vapor pressure is determined by the lowest operating temperature at any point in the arc tube and the quantity supplied is not critical. Some of the excess amalgam is needed to replace any lost during life of the lamp as it ages, for instance by electrolysis through the alumina walls.
The location where the amalgam collects in a lamp depends upon the heat balance together with the effect of gravity. In lamps having a projecting metal exhaust tube which is sealed off, the tube may provide a reservoir for excess sodium-mercury-amalgam external to the arc tube proper. Such arrangement has the advantage of placing the excess amalgam in a location removed from the direct heat of the arc and of the electrode, so that arc tube blackening as the lamp ages has a minimal effect on sodium vapor pressure and on lamp voltage. Also the use of an external reservoir facilitates close adjustment of the heat balance in the lamp, as by grit-blasting a portion of the exterior of the metal tube in order to regulate the heat loss therefrom.
Provided the heat balance in the lamp makes the external reservoir the coolest point, the excess amalgam will condense there. Capillary attraction tends to retain the amalgam where it collects, and if the lamp is operated in an attitude such that the reservoir is lowermost, gravity also helps. However, mechanical shock or heavy vibration may cause a droplet of amalgam to fly from the exhaust tube toward the hotter arc tube, particularly when the lamp's orientation places the reservoir uppermost. Vaporization of the droplet then causes a sudden rise in vapor pressure and the corresponding increase in lamp voltage may be severe enough to extinguish the lamp. When the lamp goes out in this way, commonly called drop-out, it cannot be restarted until it has cooled and that may take from 1 up to 10 minutes, depending on the ambient temperature. In extreme cases, the relatively cool droplet has been known to cause thermal cracking of the arc tube where it strikes.
Various end construction for alkali metal vapor lamps have been proposed to prevent amalgam droplets from flying out of the reservoir under adverse conditions. In U.S. Pat. No. 4,035,682--Bubar, a fine mesh screen, friction-retained in the exhaust tube, prevents passage of liquid droplets. Any droplets impinging on the screen are slowly vaporized and recondensed at the tip. In U.S. Pat. No. 4,065,691--McVey, crimping of the exhaust tube at an intermediate point leaves only restricted channels communicating with the reservoir. The channels allow passage of the amalgam in vapor form but prevent its movement as a liquid. These measures have been successful enough to allow the commercial manufacture of universal burning sodium vapor lamps suitable for ordinary applications. However they are inadequate for installations subject to really high vibration such as on highway bridges, loading docks or in the vicinity of heavy machinery.